In the preparation of emulsion cosmetics, the stabilization of the emulsion is a very important issue; thus various technologies have been developed to improve the stability (for example, Non-Patent Literatures 1 and 2).
For example, in the case of O/W (=oil-in-water type) emulsions, if liquid crystal or α-gel layers are formed around emulsion particles, aggregation/coalescence of emulsion particles are physically suppressed; thus the stability is known to improve significantly (Non-Patent Literatures 1 and 2).
α-Gel is an aggregate consisting of lamellar bi-layers (bimolecular membranes) formed by a hydrophilic surfactant and a linear higher alcohol or non-neutralized fatty acid having 16 or more carbon atoms in the presence of water (Non-Patent Literature 3). The higher alcohol and surfactant are orderly arranged in the bi-layers with the molar ratio of 3:1 (specifically, the hydrophilic group of the higher alcohol is positioned at hexagonal corners, and the hydrophilic group of the surfactant is positioned at the hexagonal center) to form a hexagonal system (Non-Patent Literature 3, pages 81 to 82). It is structurally similar to lamellar liquid crystals; however, the packing of alkyl chains is higher than lamellar liquid crystals. Therefore, blocking property is high and also excellent in the water retention; thus the above-described stabilization technology in which α-gel is used is widely used in O/W emulsion cosmetics (for example, Patent Literature 1).
However, the O/W emulsion cosmetic containing α-gel in the external phase (in the present application, it is called an α-gel-containing O/W emulsion cosmetic) has low viscosity stability over time, and it is known that the viscosity increases over time (that is, the product gradually hardens).
In addition, the cooling process of the emulsion is not easy in the production process. If the cooling rate is very slow, a large amount of α-gel is formed in the external phase and the cosmetic is too hard. If cooling is too fast, crystals of higher alcohol deposit and aggregated mass is sometimes generated. Therefore, the process is normally carried out while the cooling rate is being adjusted with a cooling device (Onlator etc.). However, a large amount of energy is necessary; in addition, a large amount of waste water is generated in the washing process of cooling devices; thus the burden is heavy on cost and the environment.
Furthermore, in the conventional production method, the preparation of an emulsion with small emulsion particle sizes (specifically, the particle size of 1 μm or less) has been known to be difficult.
Concerning the emulsion particle size among the above-described problems that the α-gel-containing O/W emulsion cosmetics have, the present inventors have shown that an α-gel-containing O/W emulsion cosmetic having the emulsion particle size of 1 μm or less can be prepared by initially preparing a W/O emulsion and subsequently inverting it into an O/W emulsion (Patent Literature 2).
Concerning the above-described burden on cost and the environment, the reduction of the burden has been attempted by preparing a high-concentration intermediate composition. In Patent Literature 3, the present inventors have reported that a α-gel-containing O/W emulsion cosmetic can be prepared without using a cooling device by preparing a high-concentration emulsion portion by the emulsification, at 70° C. or higher, of an oil phase containing a higher alcohol having 16 or more carbon atoms and a nonionic surfactant, which together can form α-gel, an oil component, and only a portion of the aqueous phase and by diluting the high-concentration emulsion portion with the residual aqueous phase at around room temperature. Patent Literature 4 discloses a method to prepare an O/W emulsion cosmetic by preparing a fine oil gel (oil-gel emulsion portion) by adding a heated oil phase with stirring to an aqueous phase, where a high-concentration water-soluble solvent and a nonionic surfactant are dissolved by heating at around 70° C., and by subsequently dispersing the oil gel in the aqueous phase (in particular, refer to Example 3 of Patent Literature 4).
However, concerning the viscosity change over time, which is the biggest problem of the α-gel-containing O/W emulsion cosmetic, no effective means of its suppression have been reported. In addition, the above-described intermediate compositions (high-concentration emulsion portion and oil-gel emulsion portion) are in a thermodynamically non-equilibrated state; thus there has been a demand for a stably storable intermediate composition.